Streaming will always incur delays, as it's recording the actual visual information from a player, So this usually means somewhere between 3 frames and 3 minutes of buffering. Poorer connections will basically not be able to stream or record video at all.
How the Duty Recorder appears to work, is to actually play back the network data, as it was originally sent, but the Duty Recorder lets you change PoV. Every player within a certain range sees the same animation and combat stat data, the only data that is unique to your character is inventory (eg when you win a loot roll, it synchronizes the inventory.)
A 24-player Alliance Raid that lasts the full 2hours, generates about 200MB of uncompressed network data. That's about the same size as a 10 minute 1080p30 video.
Just based on what I know how the game works:
1) Replacing the player ID with a NPC ID, is braindead simple. The command mission NPC's are only different from Player's and Pets in that they have a fixed "player" appearance. Replacing another player with Alphinaud is basically replacing his appearance, not his job.
2) When a character is spawned, it transmit's it's appearance. You can't change your gear during a fight, and this is why. When you change jobs, the game basically despawns and respawns your character on everyone elses screen. This also results in the stats being re-transmitted due to the gear change.
So, for using the Duty Recorder on ARR Duties, would logically let you drag Y'shtola (CNJ/WHM/AST), Yda/Lyse (DPS - MNK), Papalymo (DPS - THM/BLM), and Thancred (DPS/Rogue/Ninja) in, up until Thancred disappears. Alphinaud(ACN/SMN) and Alisaie (RDM) could also be swapped in at any point where they aren't NPC's. Yugiri (DPS - Ninja) can be added at any point in 2.1+, and used in any duty where's she's not a NPC. Likewise Gosetsu can be swapped in for any SAM. Tataru is also technically a Arcanist as well. Basically the only role that is really missing a NPC equal is Scholar. Alphinaud and Tataru could actually do all three ACN/SMN/SCH as stand-in's, I just don't know if the game would animate the pet spawns if they don't technically have them.
Like it doesn't take much to fit them in. There are Job NPC's that can also be used if they've been unlocked. The game simply needs to ask who to use (Scions/NPCs, Squadron, Randomly picked appearance) and undye/unglamor the gear. Or if we want to turn the "privacy" directive up a little more, have fixed glamours set for level 50+/60+/70+ gear so that Anonymized characters are glamoured to the AF gears.
If this was taken up to "video editing" levels, being able to overwrite the appearance/dye/glamour of the other consenting players for playback would be a good option. It might be slightly abusive if it's done without consent, so players who didn't consent would result in letting the player doing the playback "re-roll" the anonymous appearance.
To a certain extent the Duty Recorder is a superior option, if it can record everything, not just one or two fights.